Radiation-induced bystander effect

Presented by: masoud najafi

outline

• Introduction• Mechanisms of radiation-induced bystander effect• Abscopal effect• Mechanisms of abscopal effect

Bystander effect

• The bystander effect refers to the induction of biological effects in cells that are not directly traversed by radiation

• bystander effect has been demonstrated for both high- and low-LET radiations

• Bystander studies imply that the target for the biological effects of radiation is larger than the cell

First time Low doses of α particles have been shown to lead to the formation of sister chromatid exchanges in 30–50% of the cell population despite the fact that only 1% of the cells’ nuclei would have been traversed by an α particle

Bystander effect

• Bystander effects lead to: reduced clonogenic survival increased sister chromatid exchange formation of micronuclei and apoptosis altered gene expression and levels of RNA transcripts • These effects are very similar to direct effect of

radiation

mechanisms

The mechanisms of radiation-induced bystander effect that have known so far include: - immune system - gene expression - epigenetics modulators

Immune system

• cytokines, including IL-1, IL-2, IL-8, TGFß1 and TNFα • TGFß1 and TNFα stimulates apoptosis pathway• TGFß1 and TNFα produce NO and lead to elevated

stress oxidative• IL-8 stimulate MAPKs activation and ROS

production• IL-1 and IL-2 stimulate IL-8 production

Gene expression by stress oxidative

• Several genes can participate in bystander effect include:- Mitogen Activated Protein Kinases genes (MAPKs)- cyclooxygenase-2 (COX-2)- Nuclear factor of κB (NF-κB)

MAPKs genes

• MAPKs include ERK, P38, JNK and c-jun• Activated forms of the proteins belonging to the

extracellular signal-regulated kinase (ERK) and c-jun, N-terminal kinase (JNK) pathways have been demonstrated in bystander cells

• Phosphorylated forms of p38 show significant modulation in radiation-induced bystander cells

• The MAPKs activation leads to production of enzymes such as iNOS and cyclooxygenase-2

MAPKs genes

• ERK is believed to respond predominantly to mitogenic stimuli such as growth factors and enhanced calcium ions

• JNK and p38 are thought to respond to physiological stresses e.g. chemical exposure, oxidative stress and radiation

• MAPKs has been shown to be activated in response to DNA damage induced by exposure to chemicals such as mitomycin C, bleomycin and ionizing radiation

MAPK genes

Nuclear factor of κB

• Nuclear factor of κB (NF-κB) is a transcriptional regulator that is made up of different protein dimers

• this transcription factor also regulates cell proliferation and apoptosis

• NF-κB activation leads to production of enzymes such as iNOS and cyclooxygenase-2, which enhance the production of ROS, leading to additional DNA damage

• it is not surprising that NF-κB has been shown to be constitutively activated in several types of cancer cell

NF-κ B contributes to the induction of four classes of genes

cycloocigenase-2

• cyclooxygenase-2 (COX-2) signaling cascade which is essential in mediating cellular inflammatory response plays an essential role in the bystander process

• Microarray analyses of differentially expressed genes among the bystander cells showed a 3-fold overexpression of cyclooxygenase-2

• suppression of COX-2 activity in bystander cells significantly reduced the bystander effect

Epigenetics

• Epigenetics is a system that turns our genes on and off

• The process works by chemical tags, known as epigenetic marks, attaching to DNA and telling a cell to either use or ignore a particular gene

• offspring may inherit altered traits due to their parents' past experiences

mechanism of epigenetic control • DNA Methylation• Histone modifications including - acetylation - methylation - phosphorylation - Ubiquitination• miRNAs• siRNAs• piRNAs

DNA methylation

• DNA methylation is known to be associated with an inactive chromatin state

• In mammals, three DNA methyltransferases (DNMT1, DNMT3a and DNMT3b) are primarily responsible for DNA methylation

• In mammals, the association of DNA methylation with transcriptional repression is thought to be mediated by the MBD (methyl CpG binding domain)

miRNAome

• MicroRNAs are of a special interest, as they can inhibit the translation of a variety of proteins

• After the association with the RISC complexes, miRNAs bind to the 3’UTR of mRNAs and serve as translational suppressors

• Regulatory miRNAs in association with the mRNA machinery impact cellular differentiation, proliferation and apoptosis

epigenetic changes in the exposed tissue

• DNA damaging agents including Ionizing Radiation have been reported to affect DNA methylation patterns

• IR exposure leads to profound dose-dependent and sex- and tissue specific global DNA hypomethylation

• Radiation induce changes in histone methylation, specifically the loss of histone H4 lysine trimethylation

epigenetic changes in the bystander tissues

• DNA methylation is important for the maintenance of the radiation-induced bystander effect in cultured cells

• animal-based studies was shown to induce DNA damage and modulate the epigenetic effectors in distant bystander tissues

• localized cranial radiation exposure leads to the decreased levels of global DNA methylation in spleen tissue 7 months after irradiation

epigenetic changes in the bystander tissues

• Cranial irradiation lead to silencing of DNMT3a and MeCP2 in the bystander spleen tissue

• The distant bystander effect leads down regulation of DNMT3a and MeCP2

• Upregulation of the miR-29 family resulted in decreased levels of its targets DNMT3a and MCL1, consequently affecting DNA methylation and apoptosis

• irradiation significantly altered expression of miR-194, a miRNA putatively targeting both DNA methyltransferase-3a and MeCP2

miRNAs in bystander effect

Abscopal effect

• The Abscopal effect is the effect of radiation therapy sometimes observed as response of tumor masses remote from the site of irradiation

• Ionizing radiation can reduce tumor growth outside the field of radiation

• Much of the observed physiological Abscopal effect has been associated with splenic irradiation

That effect of radiation was related to diminishing of the tumor´s immune suppression and enhanced infiltration of

activated T cells affecting the tumor

mechanisms

• In this model no significant abscopal effect was found by radiation therapy combined with immunization by radiation therapy combined with immunization using IFN-gamma transfected tumor cells

• This might indicate that factors other than immunological are responsible for the radiation induced abscopal effect

• Other study showed abscopal effect apparently operates through p53 mediated mechanisms